Remember

Fertilization: The fusion of male gamete (n) with female gamete (n) forming zygote of complete number of chromosomes (2n)

Mendel's laws in light of chromosome theory

Remember

Mendel's second law (law of independent assortment of hereditary factors): When two individuals bearing one or more contrasting pairs of hereditary factors copulate, each factor is inherited independently of the other factors.

We studied in 3^rd prep that :-

- In Meiotic cell division, genes separate from each other forming gametes, but they meet again after fertilization process forming zygote of (2n) of chromosomes.

- Dominant trait appears in all members of 1^st generation at ratio 100%

- Dominant and recessive traits appear in members of 2^nd generation at ratio 3 dominant : 1 recessive

Fig. (3) The appearance of pink roses (dominant) and white rose (recessive)

 When two individuals bearing one or more pairs of contrasting genes copulate, every gene is inherited independently of the others.

 The assortment (distribution) of genes in gametes is independent because every gene exists on an independent chromosome.

Fig. (4) Independent assortment of genes

We conclude from the previous figure that:-

 Yellow colour and smooth shape of pea are from dominant traits

 Green colour and sinuous shape of pea are from recessive traits

 Each gene of each hereditary trait is inherited independently of the others

100% of the Individuals of 1^st generation are yellow-coloured and smooth-shaped

 Individuals of second generation have ratio of 1 : 3 : 3 :9

Scientist Thomas Morgan discovered in 1911 while studying the inheritance of traits in drosophila insect (fruit fly) that:-

ر - Genes of Drosophila exist on only four pairs of chromosomes

- Each chromosome pair carries hundreds of genes

- Genes on the same chromosome are inherited as a single unit

 This contradicts with Mendel's second law of the independent assortment of hereditary factors because Mendel's law states that hereditary factors (genes) are inherited independently of the other factors, while Thomas Morgan supposed that genes on the same chromosome are inherited as a single unit

 This means that Mendel's second law is not a general law.

 Morgan supposed that the linkage of genes with each other is because they exist on the same chromosome. He also supposed that the linkage force between linked genes depends on the distances between them.

Types of linkage between genes:-

It depends on the distances between genes, there are two types of linkage.

1- Complete linkage

Linkage in which two genes - on the same chromosome - are so close to each other that they cannot get separated.

Fig. (5) Complete linkage in Drosophila

 In the previous figure, we'll find that genes and are inherited as single unit and didn't separate (as Mendel's second law states)

 Complete linkage causes the constancy of genes, and hence the constancy of the inheritance of hereditary traits

2- Incomplete linkage

 In some cases, genes on the same chromosome don't stay linked together forever.

 Genes on the same chromosome separate from each other and get exchanged with genes of another chromosome during meiotic cell division; which causes the appearance of new traits in the offspring. This phenomenon is called "crossing over"

Crossing over:-

 It occurs during Prophase I stage of meiosis

Fig. (6) Crossing over phenomena

Prophase stage I

 Homologous chromosomes approach to each other forming Tetrad, which consists of 4 chromatids.

 The internal chromatids of homologous chromosomes pairs coil around each other (cross-over) at one or more points called Chiasma, where the breakage takes place.

 The internal parts of intertwined chromatids of homologous chromosomes pairs are exchanged, which is known as Crossing over

Anaphase I stage

 Pairs of homologous chromosomes separate from each other after crossing over forming two groups of chromosomes.

Anaphase II stage

 Chromatids gets separated from each other, they are called "building chromosomes". Then, they are distributed randomly among gametes.

 Chromatids which underwent crossing over are called "New chromosomes"

While chromatids which didn't undergo crossing over are called "Parental chromosomes". Thus, gametes contain new and parental chromosomes

 Crossing over is an incomplete linkage of genes which causes the change of hereditary traits with certain ratios

 Chances of crossing over occurrence increase by the increase of the distance between genes.

Fig. (7) Crossing over and chromosomes

 In the previous figure, the two new chromosomes are (GL) and (gl), crossing over result in the formation of 4 chromatids, which are:-

(GL)  Parental chromosome

(Gl)  New chromosome

(gl)  Parental chromosome

(gL)  New chromosome